Cadmium removal



Patented June 9, 1942 2,288,017 CADMIUM REMOVAL Carleton Henry Smith, -Montreal, Quebec, Canada, assignor to National Lead Company, New York, N. Y., a. corporation of New. Jersey No Drawing. Application" February" 26, 1941,

Serial No. 380,601

5 Claims. (CI. 75-63) This invention relates to the removal of cadmium from white metal scrap such as type metal, bearing metal, solder and the like. More particularly it relates to the selective removal of cadmium by the addition of sulfur to the molten metal from which the cadmium is to be removed.

When white metal scrap, such as type metal, Babbitt, bearing metals, solder and the like, consisting principally of alloys of lead, tin and antimony, are found to contain small amounts of cadmium, it is sometimes desirable to remove the cadmium without effecting the other principal constituents and without forming any substantial amount of dross particularly of tin dross. The amount of cadmium in such scrap is usually under 1%. The prior art has recommended the removal of cadmium by' volatilization out of the base metal and this was accomplished by heating the metal to a temperature over that of the boiling point of cadmium. However, by this method, very large amounts of dross were formed causing tremendous losses which, if tin were present, would be too costly to be commercially practical. For like reasons, the oxidation of cadmium from lead especially in the presence of tin is "also economically impracticable.

The present invention provides a simple, economical and efficient method for the removal of cadmium from white metalsby the use of sulfur, which reagent I have unexpectedly found removes cadmium in the form of cadmium sulfide selectively with respect to such other metals as lead; tin, antimony and copper.

In carrying out the process of the present invention, the alloy to be treated is melted, and sulfur stirred in at a fairly low temperature. After the introduction of the sulfur,'the temperature of the molten metal is raised, while stirring, to about 500 C. A pasty dross is then formed which is skimmed off and dried in any known manner. The cadmium sulfide contained in the dross will represent substantially all of the cadmium originally present in the white metal alloy treated.

, I have found that the exact temperature to be used in the practice of my invention will be dependent upon the results which are desired. Thus, cadmium maybe removed successfully at any temperature above the melting point of the metal to be purified. If, however, the operation is conducted under a temperature of about 450 0., copper, if present, will be simultaneously temoved together with the cadmium. Therefore,

perature must be elevated to above 450 C., preferably to about 500 C., as soon as the sulfur has been stirred into the metal. This prevents the formation of a dross containing copper or it prevents the copper from entering the dross which about 23% antimony, 13% tin, 0.46% cadmium and 0.5% copper were melted in a suitable kettle.

Three pounds of sulfur were now stirred into the metal and the temperature raised to about 500 C. A pasty dross formed which was skimmed off and dried in the usual manner. It was found that this dross contained in the form of cadmium sulfide, all of the cadmium which was formerly present in the metal, that the metal was entirely freed if it is not desired to remove the copper, the temof cadmium and that the tin, antimony and copper content of the treated metal was substantially unaffected. Seventy-four pounds of dross were recovered and the purified metal weighed 1,975 pounds. The results showed that about 0.32 pound of sulfur was capable of removing about 1 pound of cadmium.

Similar experiments conducted at about 450 to 500 C. on subsequent lots of similar metal consistently showed that while all the cadmium was removed, none of the copper was removed.

The efllciency of the sulfur in removing cadmium is high and depends to some extent on the care with which the sulfur is stirred into the metal, so as to prevent as far as possible the buming off of the sulfur.

It is evident from the foregoing that cadmium can be substantially completely removed by the use of sulfur and that with proper care, it is only necessary to add the stoichiometric amount of sulfur to be certain of getting a complete removal.

It is to be understood that the hereinabove description and example of the present invention have been given merely for illustrative and not limltative purposes and that there is no intention of excluding from the scope of the invention any modifications or variations lying within the skill of the art.

I claim:

1. The process of removing cadmium from white metals, which comprises melting the metal to be treated, admixing therewith sulfur to form a dross containing cadmium and skimming off the dross from the purified metal.

2. The process of removing cadmium from about 500 C. until the desired removalof the cadmium is eflected by the formation of a dross.

and skimming oil the dross from the'purified metal.

. 4. The process of selectively removing cadmium' from secondary lead alloys, which comprises melting the alloy to be treated, stirring therein sulfur at a relatively low temperature, raising the temperature of the molten mass to between about 450 C. and about 500 C. until the desired removal of the cadmium is effected by the formation of a dross, and skimming off the dross from the purified metal.

5. The process of selectively removing cadmium from secondary lead alloys which comprises melting the alloy to be treatedat a temperature below about 450, thoroughly stirring therein sulfur, gradually raising the temperature of the molten mass to about 500 C. until the desired removal of the cadmium is effected 'by the formation of a dross and skimming off the dross from the purifled metal.

CARLETON HENRY SMITH. 

